When a watch receives a severe shock dial feet can sheer off, pivots of wheels can break, screws can become loose and bridges can shift. All of these outcomes severly affect the time keeping of the watch and may result in catastrophic failure.
The watch industry provides testing guidlines for what it considers severe shock. The fundamental requirement of ISO 1413 is that an automatic watch, falling 1m (3.28ft) onto a hardwood floor, should maintain accuracy within a range of +/- 60 seconds/day.
There are two major issues with ISO 1413. First, the force of a 1m drop onto a hard wood floor is approximately 5000 G's. While this number may seem impresssive, high G-forces can be generated easily by common solid objects falling insignificant heights onto hard surfaces. While large forces over a long period of time (a fighter pilot maneuver) result in lower “G” numbers. The force of a golf swing on your wrist is approximatley 5000 G's. . . hardly a "high-shock" event.
Second, a watch passing ISO 1413, need only be accurate to +/- 60 seconds per day. An automatic watch with these specs would be considered broken and in need of repair.
ISO 1413, clearly, is not rigourous enough.